1. Field of the Invention
The present invention relates to an image processing apparatus and, more particularly, to an image processing apparatus for performing image formation using signals obtained by pulse-width modulating an image signal by a plurality of modulation methods.
2. Related Background Art
Conventionally, an image forming system constituted by a controller for receiving image data from a host computer and supplying received image data to an image forming apparatus, and the image forming apparatus for performing image formation based on the image data sent from the controller has been proposed.
For example, image forming systems which use a color copying machine as the image forming apparatus, and combine it with various types of controllers are commercially available.
The color copying machine serving as the image forming apparatus constituting such conventional system comprises a laser type color electrophotographic printer, which frame-sequentially performs image formation of a plurality of output color components C (Cyan), M (Magenta), Y (Yellow), and K (blacK), and expresses a halftone image by driving a laser using a signal obtained by pulse-width modulating an image signal.
The conventional color copying machine has, as a pulse-width modulation method, a first method of performing pulse-width modulation in units of pixels, and a second method of performing pulse-width modulation in units of certain numbers of pixels.
In the first method, a high resolution can be attained since a pulse is output for each pixel. On the other hand, in the second method, since a pulse is output for each plurality of pixels, the resolution lowers, but the change in pulse width amount with respect to the change in image data becomes larger than that in the first method, and the change in image data can be faithfully reproduced, thus obtaining high gradation characteristics.
As described above, the conventional color copying machine has a resolution of 400 dpi. In the first method, since pulse-width modulation is performed for each pixel, and the screen frequency corresponds to 400 lines per inch, the first method is called a 400 lines mode. On the other hand, in the second method, since pulse-width modulation is performed for each two pixels, and the screen frequency corresponds to 200 lines per inch, the second method is called a 200 lines mode.
In the conventional system, whether image formation is performed in the 200 or 400 lines mode is switched depending on the types of connected controllers, and when a print operation is performed using a single controller, one of the two modes is fixed. More specifically, when a controller which mainly aims at printing character images is used, image formation is always performed in the 400 lines mode, and when a printer which mainly aims at printing gradation images such as photographs is used, image formation is performed in the 200 lines mode.
However, in the conventional system, since the 200 and 400 lines modes are switched depending on the types of controllers, when a character image is output from a controller that performs image formation in the 200 lines mode, the resolution lowers; when a gradation image is output from a controller that performs image formation in the 400 lines mode, the gradation characteristics deteriorate.
The present invention has been made in consideration of the above situation, and has as its object to provide an image processing apparatus which can satisfactorily reproduce both character and gradation images by performing pulse-width modulation processing corresponding to a given condition in the apparatus upon image formation of an input image signal on a recording medium.
It is another object of the present invention to provide an image processing apparatus in which a controller supplies a mode signal of pulse-width modulation processing to an image forming apparatus in correspondence with a given condition, and the image forming apparatus performs image formation by executing the pulse-width modulation processing in accordance with the received mode signal, thus satisfactorily reproducing both character and gradation images.
In order to achieve the above objects, according to a preferred aspect of the present invention, an image processing apparatus comprises input means for inputting an image signal, generation means for generating a modulation mode signal upon designation by an operator using an operation unit, and modulation means for selectively outputting one of a pulse-width modulated signal obtained by performing pulse-width modulation of the input image signal in units of m pixels and a pulse-width modulated signal obtained by performing pulse-width modulation of the input image signal in units of n pixels (m less than n) in accordance with the modulation mode signal generated by the generation means.
According to another preferred aspect of the present invention, an image processing apparatus comprises image input means for inputting an image signal, modulation mode input means for inputting a modulation mode signal from a device outside the apparatus, and modulation means for selectively outputting one of a pulse-width modulated signal obtained by performing pulse-width modulation of the input image signal in units of m pixels and a pulse-width modulated signal obtained by performing pulse-width modulation of the input image signal in units of n pixels (m less than n) in accordance with the modulation mode signal input by the modulation signal input means.